1. Field of the Invention
The present invention relates to an FSK (Frequency Shift Keying) signal detector for detecting an FSK signal, and more particularly, to an FSK signal detector for detecting an FSK signal through digital processing.
2. Description of the Related Art
FIG. 19 illustrates an example of conventional FSK signal detector for detecting an FSK signal through digital processing. In this FSK signal detector, a quadrature detector circuit comprising an oscillator 100, a π/2 phase shifter 102, and multipliers 104, 106 is used to decompose a received IF signal 118 input thereto into baseband signals, i.e., I signal and Q signal, through quadrature detection. Then, A/D converters (ADC) 108, 110 are used to quantize the I, Q signals, respectively, to generate digital data representing amplitude information. A phase detector 112, for example, having a table indicating the relationship between the amplitudes of the I, Q signals to tan−1θ is used to generate phase information corresponding to the amplitudes of the I, Q signals. Then, a one-symbol delay circuit 114 and a phase transition determination circuit 116 are used to obtain a one-symbol delay difference of the phase information generated from the amplitudes of the I, Q signals to detect how the phase has changed, and outputs a detection signal 120.
For example, when a phase difference θ2−θ1 is positive, where θ1 represents the phase at a symbol S1, and θ2 represents the phase detected at a symbol S2 one symbol after the symbol S1, the phase has advanced during a transition from the symbol S1 to the symbol S2, resulting in a higher frequency at the symbol S2 as compared with that at the symbol S1. On the other hand, when a phase difference θ3−θ1 is negative, where θ1 represents the phase at the symbol S1, and θ3 represents the phase detected at a symbol S3 one symbol after the symbol S1, the phase has delayed during a transition from the symbol S1 to the symbol S3, resulting in a lower frequency at the symbol S3 as compared with that at the symbol S1. In this way, the FSK signal detector in FIG. 19 detects the frequency of the FSK signal 118 by converting a phase shift between symbols to a frequency shift. Reference can be made to, for example, Japanese Patent Laid-Open Publication Kokai No. H09-36924.
However, the foregoing FSK signal detector detects the frequency on the basis of the amplitude information of the I, Q signals, fluctuations in the amplitudes of the I, Q signals due to noise and the like, if any, would adversely affect the characteristic of the frequency detection. Also, the FSK signal detector is problematic in that it needs such parts as A/D converters, and involves a complicated circuit configuration because the amplitude information of the I, Q signals is represented using a plurality of bits.